1 Comment

Chapter 38 – Optics of Aphakia and Pseudophakia

Section 3 – Lens replacement

 

 

283

Chapter 38 – Optics of Aphakia and Pseudophakia

 

JACK T. HOLLADAY

 

INTRODUCTION

The normal 72-year-old human eye has a total dioptric power of approximately 58D, with nearly 75% of the power from the cornea and 25% of the power from the crystalline lens ( Fig. 38-1 ).[1] Removal of the crystalline lens leaves the eye extremely deficient in dioptric power, which must be replaced to restore vision. The replacement of the dioptric power can be in the form of spectacles, contact lenses, corneal onlays, corneal implants, or intraocular lenses. Although each modality can restore the patient’s vision, the optical consequences are dramatically different and must be understood by the clinician to avoid unnecessary complications.

HISTORICAL REVIEW

The most common and successful method to replace crystalline lens power is to use an intraocular lens (IOL). The earliest documented IOL implant was performed by Harold Ridley in 1949.[2] Ridley’s original IOL was made of polymethyl methacrylate (PMMA) and placed in the posterior chamber, in a manner very similar to that of the present method. Over the past 50 years, improvements in the purity of the PMMA, in the quality

 

 

Figure 38-1 Standardized 72-year-old phakic eye. The values shown are the mean values for a phakic eye: keratometric power of the cornea (kker ), net refractive power of the cornea (kref ), and anterior radius of the cornea (rant ). Indices of refraction (n) are 1.336 for the aqueous and vitreous (neye ) and 1.000 for air (nair ).

of lens manufacturing, and in the surgical techniques used have transformed this technique into one of the most successful surgical procedures performed today.

TREATMENT

Aphakia

Figure 38-2 shows the aphakic eye with a spectacle lens at a vertex of 14?mm to correct the patient’s vision. Replacement of the crystalline lens power with a spectacle lens causes the image that is formed on the patient’s retina to be roughly 25% larger than the image formed with the crystalline lens. The actual magnification is determined by the exact power of the aphakic spectacles. There is approximately 2% of magnification for each diopter of power in the spectacles. The average aphakic spectacle is therefore 12.5D.

The magnification from aphakic spectacles causes other optical aberrations, such as a ring scotoma ( Fig. 38-3 ), jack-in-the-box phenomenon ( Fig. 38-4 ), and a pincushion distortion ( Fig. 38-5 ). Because the image through the spectacles is magnified by 25%, the actual field of view through the spectacles is reduced by 25%, which makes it impossible to see the 25% of the peripheral

 

 

Figure 38-2 Standardized 72-year-old aphakic eye. The values shown are the mean values for a phakic eye: keratometric power of the cornea (kker ), net refractive power of the cornea (kref ), and anterior radius of the cornea (rant ). Indices of refraction (n) are 1.336 for the aqueous and vitreous (neye ) and 1.000 for air (nair ).

 

284

 

 

Figure 38-3 Ring scotoma. An area of about 9° of the field. The blind wedge extends around the circumference, hence the term ring scotoma.

 

 

Figure 38-4 The jack-in-the-box phenomenon. A, Ring scotoma of an aphakic patient plotted by perimetry. The roving ring scotoma shifts centrally as the eye rotates peripherally. The result is the jack-in-the-box phenomenon, in which an object is seen with peripheral vision, the eye turns to look directly at the object, and the object disappears. Aphakic correction is by spectacle lenses. B, Perimeter of an aphakic eye that has a contact lens worn for correction of aphakia. Contact lenses remove the jack-in-the-box phenomenon entirely.

 

 

Figure 38-5 Distortion from spectacle lenses with oblique angles of gaze.

field that would be seen normally through spectacles with no power. The result is an annulus of no vision, or ring scotoma.

When the image of an object moves from the extreme visual field toward the center of fixation, as it passes through the ring scotoma it disappears until it moves into the central island of vision. This jumping into and out of the patient’s vision has been referred to as the jack-in-the-box phenomenon. [3] [4] [5] Driving a motor vehicle thus becomes very difficult to perform, as does any activity in which objects move rapidly across the visual field.

Pincushion distortion is a property of all plus lenses and is proportional to their dioptric power. This distortion makes a square look like a pincushion—the corners of the square have a stretched-out appearance, and the sides are pushed in, as shown in Figure 38-5 . Every object viewed through aphakic spectacles is distorted in this way, which makes rectangular objects, such as doors and boxes, appear like a pincushion. For an architect or draftsman, these distortions make the job extremely difficult or impossible to perform. The distortions created by aphakic spectacles necessitated the development of other modalities, such as IOL and corneal onlays or inlays.

Corneal Contact Lenses, Onlays, and Inlays

To correct aphakia at the corneal plane involves the use of contact lenses or surgery that adds dioptric power to the cornea. As the position at which the optical correction is made moves

 

285

 

 

Figure 38-6 Standardized 72-year-old pseudophakic eye (thin IOL). The values shown are the mean values for a pseudophakic eye: keratometric power of the cornea (kker ), net refractive power of the cornea (kref ), and anterior radius of the cornea (rant ). Indices of refraction (n) are 1.336 for the aqueous and vitreous (neye ) and 1.000 for air (nair ). Using these values, the required thin IOL power is 21.19D at an effective lens position (ELP) of 5.25?mm.

closer to the retina, the necessary dioptric power increases but the subsequent magnification decreases. The power at the corneal plane that is equivalent to 12.5D at a vertex of 12?mm is 14.7D; a patient who needs 12.5D in aphakic spectacles would need 14.7D in a soft or rigid contact lens. At the corneal plane the magnification is 6–8%. This value is near the limit of aniseikonia (image size disparity between the two eyes),[6] [7] so most unilaterally aphakic patients can have binocular vision, with the aphakic eye corrected using a contact lens and the other eye phakic. Binocular vision is not possible with one aphakic spectacle and a normal phakic lens.

Corneal onlays, such as epikeratophakia, and inlays, such as the intracorneal lenticle, are still in the early phases of investigation and are not used commonly in clinical settings. The optical effects are no different from those of a contact lens, but onlays and inlays have the advantage that the patient need provide no maintenance. However, the excellent success of contact lenses and IOLs means that surgical techniques to correct aphakia at the cornea are not reasonable clinical alternatives at this time.

Pseudophakia

Figure 38-6 shows a posterior chamber lens in-the-bag following cataract extraction. Just as the average spectacle power for aphakia is 12.5D, the average power of an equiconvex IOL in-the-bag is approximately 21D. The average magnification of an IOL in this position is 1.5%, compared with the original crystalline lens. For an anterior chamber IOL the average power would be less, approximately 18D, and the magnification would be approximately 2.0%. Although some discerning patients can detect this disparity by alternately covering each eye, almost everyone can achieve binocular vision with one eye pseudophakic and the other phakic.[8]

PSEUDOPHAKIC LENS DESIGN.

The IOLs currently available are either biconvex, convexoplano, or meniscus. As a result of clinical performance and optical analysis, the majority of lenses implanted today are biconvex.[9] [10] The reasons for the emergence of this design as the most superior are both optical and mechanical.

The quality of the optical design of an IOL is measured on the basis of its performance with respect to tilting, decentration, and spherical aberration. In terms of each of these, the positive meniscus lens performs miserably and rarely is used today. The original design concept was to create a “laser space,” so that the posterior surface of the lens would not be in contact with the posterior capsule; this avoids pitting of the lens with yttrium–aluminum–garnet laser capsulotomy. When a meniscus lens is tilted or decentered, the induced astigmatism and power change are dramatic. A 10–15° tilt can induce enough regular and irregular astigmatism to make the spectacle correction intolerable and results in a best corrected vision of less than 20/20 (6/6), simply because of the poor optics.

Convexoplano IOLs (convex on the front surface and flat on the posterior surface) were the first to be designed. They are the simplest to manufacture, because one surface is flat and all the optical power lies in the other surface. These lenses have performed well over the years, but degradation of the retinal image with lens tilt or decentration is still greater than it is with biconvex lenses. Optical studies to determine the optimal lens design have shown that a biconvex design with a front surface much steeper than the back appears to minimize this aberration for most humans.[11] No clinical studies have demonstrated a difference in the spherical aberration of a convexoplano lens with respect to that of a biconvex lens that is steeper on the front surface.

The optimal optical and mechanical performance of an IOL in the human eye is that of biconvex lenses. In addition to minimizing the effects of tilt, decentration, and spherical aberrations, a convex posterior surface often reduces the migration of lens epithelial cells, a migration that may lead to opacification of the capsule; this is an additional mechanical advantage of biconvex over convexoplano lenses. The biconvex IOL has become the predominant lens style used today because of its superior optical and mechanical clinical performance.[9]

EDGE DESIGN.

Reflections, shimmering peripheral lights, and flashes usually are related to the edge design of a lens. Flat edges from truncation (oval optics) or flat edges in round optics create unwanted external and internal reflections that the patient may see in low light levels.[12] Therefore, most lenses have rounded edges to avoid a coherent reflected image from the edge of a lens’ flat surface. Oval (e.g., 5 × 6?mm) optics flourished for a short time as a result of the desire to insert lenses through smaller and smaller incisions, until studies demonstrated an increased incidence of unwanted peripheral reflections from these lenses. It was shown that these unwanted reflections came from the flat, truncated edge of the lens.[12] Flat edges are no longer used in modern IOLs.

OPTICAL TRANSMISSION.

The optical transmission through the human eye to the retina usually is considered to be in the range 400–700?nm in wavelength. The cornea filters any wavelength shorter than 300?nm, and the crystalline lens filters out any wavelength shorter than 400?nm. When the crystalline lens is removed, wavelengths of 300–400?nm reach the retina. PMMA that is not treated specially filters only light below 320?nm, so in the late 1970s much discussion ensued as to whether the 320–400?nm wavelengths that reached the pseudophakic retina could cause syneresis of the vitreous, macular degeneration, cystoid macular edema, and erythropsia.

Manufacturers began to modify PMMA to filter out wavelengths below 400?nm (ultraviolet light [UV]), just as the crystalline lens does, to protect the retina. Clinical studies have found that this additional filter has no effect on any of these conditions except erythropsia (in which vision appears to be through a red transparency because blue cones have been bleached out by excessive UV). As a result, very few non–UV filtering IOLs are manufactured today.

MATERIAL.

Commercially available IOLs are made of PMMA, silicone, or acrylic. Lenses made of hydroxyethyl methacrylate

 

286

still are under investigation. The discussion here is limited to commercially available lenses. Silicone and acrylic lenses are foldable, so they can be implanted through small incisions (less than 3.5?mm in length). The index of refraction for PMMA is 1.491, that for silicone is in the range 1.41–1.46, depending on the model and manufacturer, and for acrylic it is 1.55. The higher the index of refraction, the flatter the curvatures of the lens need to be to achieve the same refractive power. For a 20D biconvex IOL with 10D on each surface, the acrylic lens has the flattest curvatures and the silicone the steepest. As a consequence of the flatter curvatures, the acrylic lens is thinner than the PMMA lens which, in turn, is thinner than the silicone lens, provided all else is equal.

The velocity of ultrasound for these materials at eye temperature (35°C) is 2658?m/sec for PMMA, 980–1090?m/sec for silicones, and 2180?m/sec for acrylic.[13] All three of these lens materials have performed well clinically, although the long-term results (over 10 years) are still awaited for silicone and acrylic lenses.

Specialty Intraocular Lenses

Two other special types of IOLs are manufactured currently— multifocal and toric. Multifocal IOLs have enjoyed a success similar to that of multifocal contact lenses. Multifocal IOLs produce two or more focal points, which create a focused and defocused image on the retina. The result is an image that is approximately 30% reduced in contrast with respect to monofocal lenses and unwanted optical images seen at night, such as halos or rings around headlights.[14] The reduced image quality must be weighed against the patient’s desire to be less spectacle dependent. With (multifocal) contact lens failure, the problem is solved by returning to spectacles. A patient who is dissatisfied with a multifocal IOL is more difficult to deal with, and lens exchange occasionally is required. The success of these lenses is based almost entirely on appropriate patient selection.

Toric IOLs are simply spherocylindric lenses, just like spectacles. If the toric lens is aligned properly with the patient’s corneal astigmatism and the magnitude is correct, the patient’s corneal astigmatism can be neutralized. The magnitude of the cylinder in the IOL must be approximately 1.4 times the astigmatism in the cornea to neutralize completely the corneal astigmatism; for corneal astigmatism of 1.0D, the cylinder in the IOL must be 1.4D. Manufacturers usually provide two nominal toricities and recommend using the one that best fits the particular patient on the basis of a nomogram. As long as the lens is within 30° of the intended axis, the patient has less astigmatism in the spectacles than in the cornea. If the lens is misaligned by more than 30°, the patient has greater astigmatism in the spectacles than in the cornea. It is obvious that the lens must fixate well and not rotate from the axis of the original correct placement, otherwise the patient’s refraction fluctuates and the benefit of a toric lens diminishes.

 

 

REFERENCES

 

1. Campbell CJ, Koester CJ, Rittler MC, Tackaberry RB. The optics of the eye. In: Physiological optics. Hagerstown: Harper & Row; 1974:99–110.

 

2. Ridley H. Intraocular acrylic lenses. A recent development in the surgery of cataract. Br J Ophthalmol. 1952;36:113–22.

 

3. Michaels DD. Aphakia and pseudophakia. In: Michaels DD. Visual optics and refraction. St Louis: Mosby; 1985:506–27.

 

4. Rubin ML. Optics for clinicians, ed 2. Gainesville: Triad; 1974:249–54.

 

5. Milder B, Rubin ML. Aphakia. In: Milder B, Rubin ML. The fine art of prescribing glasses, ed 2. Gainesville: Triad; 1991:283–309.

 

6. Milder B, Rubin ML. Anisometropia. In: Milder B, Rubin ML. The fine art of prescribing glasses, ed 2. Gainesville: Triad; 1991:217–53.

 

7. Burian HM, von Noorden GK. Visual acuity and aniseikonia. In: Binocular vision and ocular motility. St Louis: Mosby; 1974:130–41.

 

8. Holladay JT, Rubin ML. Avoiding refractive problems in cataract surgery. Surv Ophthalmol. 1988;32:357–60.

 

9. Holladay JT, Prager TC, Bishop JE, Blaker JW. The ideal intraocular lens. CLAO J. 1983;9:15–9.

 

10. Holladay JT. Evaluating the intraocular lens optic. Surv Ophthalmol. 1986;30: 385–90.

 

11. Atchison DA. Optical design of intraocular lenses. I. On-axis performance. Optom Vision Sci. 1989;66:492–506.

 

12. Masket S, Geraghty E, Crandall AS, et al. Undesired light images associated with ovoid intraocular lenses. J Cataract Refract Surg. 1993;19:690–4.

 

13. Yang S, Lang A, Makker H, Azleski E. Effect of silicone sound speed and intraocular lens thickness on pseudophakic axial length corrections. J Cataract Refract Surg. 1995;21:442–6.

 

14. Holladay JT, Van Dijk H, Lang A, et al. Optical performance of multifocal intraocular lenses. J Cataract Refract Surg. 1990;16:413–22.

Advertisements

One comment on “Chapter 38 – Optics of Aphakia and Pseudophakia

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: